City Of Tacoma Central Wastewater Treatment Plant

1. INTRODUCTION

The Central Wastewater Treatment Plant (CTP) is the cornerstone of the City of Tacoma’s wastewater infrastructure and a critical environmental safeguard for the Puget Sound region. Treating approximately 80% of the city’s wastewater, the facility manages an average dry weather flow of 23 million gallons per day (MGD), with a robust hydraulic design capable of handling peak wet weather flows up to 150 MGD. This high peaking factor is necessitated by portions of Tacoma’s collection system being Combined Sewer Systems (CSS), requiring the plant to manage significant stormwater intrusion during Pacific Northwest rain events.

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Operated by the City of Tacoma Environmental Services Department, the CTP is distinguished by its High Purity Oxygen (HPO) activated sludge process and its nationally renowned biosolids program, TAGRO (Tacoma Grow). The facility produces Class A Exceptional Quality biosolids, setting a benchmark for circular economy practices in municipal wastewater treatment. Following significant upgrades over the last decade totaling over $100 million, the CTP maintains a record of operational excellence, frequently earning Platinum Peak Performance Awards from the National Association of Clean Water Agencies (NACWA).

2. FACILITY OVERVIEW

A. Service Area & Coverage

The CTP serves a highly urbanized and industrial service area covering the majority of the City of Tacoma, as well as providing wholesale treatment services for the neighboring jurisdictions of Fife, Fircrest, and unincorporated areas of Pierce County. The collection system feeding the CTP is complex, comprising over 700 miles of wastewater mains and 46 pump stations. Roughly 28% of the collection system remains a combined system, which heavily influences the plant’s operational strategy during storm events.

B. Operational Capacity

The facility demonstrates significant hydraulic flexibility:

• Design Peak Hydraulic Capacity: 150 MGD
• Design Max Month Flow: 60 MGD
• Average Annual Flow: ~20-28 MGD

Historical flow trends indicate a stabilization of dry weather flows despite population growth, attributed to aggressive inflow and infiltration (I/I) reduction programs and water conservation efforts. However, the “first flush” phenomena during heavy rainfall requires rapid ramping of the High Purity Oxygen systems to maintain biological viability under high hydraulic loading.

C. Discharge & Compliance

Treated effluent is discharged into Commencement Bay via a deep-water marine outfall equipped with a diffuser assembly to maximize initial dilution. The receiving water is a sensitive marine environment within Puget Sound, subject to strict oversight by the Washington State Department of Ecology. The plant operates under NPDES Permit WA0037087, which mandates rigorous monitoring of BOD, TSS, pH, and residual chlorine. The facility consistently achieves high removal efficiencies, typically exceeding 90% removal for both TSS and BOD5.

3. TREATMENT PROCESS

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The Tacoma CTP utilizes a High Purity Oxygen (HPO) activated sludge process, selected for its ability to treat high-strength waste within a smaller physical footprint compared to conventional air aeration. The treatment train is detailed below:

A. PRELIMINARY TREATMENT

Raw influent enters the headworks where it passes through mechanically cleaned bar screens to remove large debris, rags, and plastics. Following screening, the flow enters aerated grit chambers where inorganic solids (sand, gravel) settle out. The grit is removed, washed, and dewatered for landfill disposal. During peak wet weather events exceeding 60 MGD, the plant can engage specific hydraulic protocols to bypass excess flows around secondary treatment if necessary, though the 2008 upgrades significantly expanded full secondary treatment capacity.

B. PRIMARY TREATMENT

Flow proceeds to rectangular primary sedimentation tanks. These basins reduce the velocity of the wastewater, allowing settleable organic solids to drop to the bottom as primary sludge, while grease and oils float to the surface for skimming. Primary treatment typically removes 50-60% of Total Suspended Solids (TSS) and 30-40% of Biochemical Oxygen Demand (BOD). The primary sludge is pumped directly to the solids handling processes (TAGRO).

C. SECONDARY TREATMENT (High Purity Oxygen)

The core of the CTP is the HPO activated sludge system. Unlike conventional aeration basins that blow ambient air (21% oxygen) into the water, the CTP utilizes covered reactors where high-purity oxygen (>90%) is introduced.

• Oxygen Generation: The plant utilizes on-site Vacuum Pressure Swing Adsorption (VPSA) or cryogenic oxygen generation systems to produce pure oxygen.
• Reactor Configuration: The basins are covered to maintain oxygen partial pressure, significantly increasing the oxygen transfer rate (OTR) and allowing for higher mixed liquor suspended solids (MLSS) concentrations.
• Biological Process: This high-rate environment promotes a robust biomass capable of handling shock loads common in combined sewer systems.
• Secondary Clarification: Mixed liquor flows to secondary clarifiers where the biomass settles. Return Activated Sludge (RAS) is recycled to the head of the HPO basins, while Waste Activated Sludge (WAS) is sent to solids handling.

D. DISINFECTION

The clarified effluent undergoes disinfection using sodium hypochlorite (liquid chlorine). The flow passes through chlorine contact channels designed to ensure adequate contact time (CT) for pathogen inactivation. Prior to discharge into Commencement Bay, the effluent is dechlorinated using sodium bisulfite to protect marine life from chlorine toxicity.

F. SOLIDS HANDLING (TAGRO Production)

The CTP is famous for its award-winning TAGRO (Tacoma Grow) biosolids program. The solids handling process is engineered to produce Class A Exceptional Quality (EQ) biosolids.

• Dual Digestion: The plant utilizes a temperature-phased anaerobic digestion process or a dual-stage process. This involves varying thermal regimes to ensure pathogen destruction (meeting Class A vector attraction reduction requirements) and volatile solids reduction.
• Biogas Utilization: Methane produced during digestion is captured and used to heat the digesters and facility buildings, contributing to energy neutrality goals.
• Dewatering: Digested sludge is dewatered using centrifuges or belt filter presses to increase solids content.
• Product Formulation: The dewatered cake is processed into various soil amendment products (TAGRO Mix, TAGRO Potting Soil) available to the public and used in municipal parks.

4. INFRASTRUCTURE & FACILITIES

A. Physical Plant

Situated in the industrial tidal flats of Tacoma, the site is constrained by the Puyallup River and urban development, necessitating the compact footprint provided by the HPO process. The facility includes extensive maintenance shops, a certified environmental laboratory, and the TAGRO distribution center.

B. Energy Systems

The CTP is a significant energy consumer, primarily due to the oxygen generation systems and influent pumping. However, the facility mitigates this through the capture and beneficial use of digester gas (biogas). The plant utilizes boilers to recover heat from biogas for process heating. Tacoma Power, the local utility, provides hydroelectric-sourced grid power, giving the plant a relatively low carbon footprint regarding Scope 2 emissions.

C. Odor Control

Given the facility’s proximity to commercial areas and the I-5 corridor, odor control is paramount. The CTP employs chemical scrubbers (wet scrubbers) and carbon adsorption units to treat foul air extracted from the headworks, primary clarifiers, and solids handling buildings.

5. RECENT UPGRADES & MAJOR PROJECTS

6. REGULATORY COMPLIANCE & ENVIRONMENTAL PERFORMANCE

The CTP operates under strict regulatory frameworks focused on the health of Puget Sound.

A. Permit Requirements

Under NPDES Permit WA0037087, the facility must meet limits for:

• BOD5: Monthly average ≤ 30 mg/L (or 85% removal, whichever is more stringent)
• TSS: Monthly average ≤ 30 mg/L (or 85% removal)
• Fecal Coliform: Geometric mean ≤ 200 colonies/100 mL
• pH: 6.0 to 9.0 standard units

B. Compliance History

The City of Tacoma has an exemplary compliance record. The CTP has received the NACWA Platinum Peak Performance Award multiple times, recognizing facilities with five or more consecutive years of 100% compliance with NPDES permit limits. This reliability is particularly notable given the variable loading inherent in the combined sewer system.

7. CHALLENGES & FUTURE PLANNING

A. Nutrient Management (Nitrogen)

The primary challenge facing the CTP (and all Puget Sound dischargers) is the “Puget Sound Nutrient General Permit.” Reducing nitrogen requires nitrification and denitrification biological steps. Retrofitting an existing HPO plant for high-level nitrogen removal is technically complex due to carbon limitations and footprint constraints. The City is currently modeling strategies to maximize nitrogen removal within existing infrastructure while planning for potential future expansion.

B. Climate Resilience

Located at sea level near a tidal river, the CTP is vulnerable to sea-level rise and storm surges. Future master planning includes hardening the facility against higher flood elevations and ensuring pumping capacity can overcome higher hydraulic heads at the outfall during king tides.

C. Aging Infrastructure

While the Peak Flow facility is relatively new, core components of the 1980s expansion—specifically the oxygen generation train and older clarifier mechanisms—require ongoing asset management and systematic replacement.

8. COMMUNITY & REGIONAL IMPACT

The CTP’s most visible link to the community is the TAGRO program. By converting wastewater solids into high-value potting soil and mulch, the plant closes the loop on waste. The program generates revenue for the utility, reduces landfill costs, and improves local soil health. TAGRO products are widely used in community gardens, personal lawns, and by the City’s Parks Department, fostering a positive public perception of wastewater treatment operations.

9. TECHNICAL SPECIFICATIONS SUMMARY

10. RELATED FACILITIES

• North End Treatment Plant (NETP): A separate physical-chemical treatment plant serving the north end of Tacoma, utilizing chemically enhanced primary treatment and filtration.
• Central Pump Station: The primary lift station conveying flows to the CTP headworks.

11. FAQ

Technical Questions

Q: Why does Tacoma CTP use High Purity Oxygen (HPO)?
A: HPO was selected to handle high organic loads in a smaller footprint than conventional air activated sludge. The enclosed reactors also provide better control over odors in the urban setting.

Q: Does the CTP perform biological nutrient removal (BNR)?
A: Historically, the plant was designed for BOD and TSS removal. Currently, the plant is undergoing optimization to enhance nitrogen removal in response to the new Puget Sound Nutrient General Permit.

Q: How does the plant manage peak wet weather flows?
A: The plant utilizes a specialized peak flow treatment train authorized under its permit to handle flows exceeding secondary capacity, providing primary treatment and disinfection before blending with secondary effluent.

Public Interest Questions

Q: Is TAGRO safe for vegetable gardens?
A: Yes. TAGRO products are Class A “Exceptional Quality” biosolids, meaning they have been treated to kill pathogens and meet strict EPA standards for heavy metals. They are safe for use in vegetable and flower gardens.

Q: What happens to the water after treatment?
A: The treated, disinfected, and dechlorinated water is discharged through a deep-water outfall into Commencement Bay, roughly 1,000 feet offshore.